Lenses formed of plastic material for optical applications are well known. A primary use for such lenses is as contact lenses. Generally, contact lenses fall into two major categories one of which is generally characterized as soft hydrophilic contact lenses and the other of which is characterized as rigid gas permeable or hard contact lenses which are generally hydrophobic. In addition to characterization of lenses as "soft" or "hard", contact lenses are often classified as corneal or scleral. A corneal lens is a contact lens whose main bearing portion rests upon the cornea of the eye. Another specialized type of lens is the intraocular lens which is surgically implanted into the eye.
Because of the stringent requirements for first quality contact lenses, extreme precision is required in the making of these lenses. Most plastic contact lens blanks are formed by initially casting an elongated cylindrical rod from a plastic material, such, for example, as cellulose acetate butyrate, silicone acrylate, fluorosilicone acrylate, polymethyl methacrylate, and the like. The cylindrical rod is then transversely cut to form a number of cylindrical lens blanks or buttons. The blanks, having generally opposite planar surfaces, are thereafter furnished to the lens manufacturer where each is machined to prescription and thereafter shipped for fitting to the patient by the fitter. Various machining operations may be accomplished on the blanks. For example, it is common practice to machine the lens blank using a lathe equipped with a diamond bit or other machine tool such as a spherical rotating tool bit. However, the machining operation will impart markings in the surface of the lenses which impairs optical quality and the lens surface must be therefore be very carefully polished to remove the machined surface markings. Underpolishing will leave lathe lines on the optical surface while overpolishing will cause orange peel and a poor quality optical surface.
Another problem in forming lenses from elongated rods is that it is difficult to fabricate such rods having a uniform density. A non-uniform density in the rod creates considerable optical problems such as variations in the refractive index and in the mechanical strength of the manufactured lens.
In an effort to avoid some of the problems of which are inherent in manufacturing plastic contact lens blanks from sections cut from plastic rods, the prior art suggests the formation of contact lenses by pouring the plastic between two parallel spaced glass sheets to form therebetween a plastic sheet having clear surfaces. The plastic sheet is then cut into squares that are slightly larger than the desired diameter of the circular lens blanks. The square pieces are positioned between spindles and are rotated while the periphery is machined by a cutting tool to the desired diameter. Such a procedure is described in U.S. Pat. No. 3,651,192.
Attempts have been also made to cast high quality plastic lenses. For instance, U.S. Pat. No. 3,380,718 suggests the use of a lower concave mold which is filled with allyl diglycol carbonate. A convex mold is lowered by a centering rod mechanism into the material. Heat is applied until the liquid is in the gel stage. Pressure is further increased after the gel stage to reduce shrinkage and other undesirable characteristics. While this process represents an advance in the state of the art, it is still generally limited to special types of lenses, such as bifocal lenses. It is clearly not applicable to contact lenses or to the various plastic materials conventionally used to produce contact lenses. Warpage as well as shrinkage frequently occurs with this method.
Another attempt at improving the methodology of casting contact lens was presented by Neefe in U.S. Pat. No. 4,457,880 in which a lens blank having a finished optical surface is cast from a liquid monomer beneath an optical-surfaced upper mold which is made from a resinous material which adheres to the upper surface of the polymerized lens material. While this approach provided a "handle" for the resulting lens blank while it was being machined, it did nothing to reduce internal stresses on the lens or facilitate shipment and carried with it the further risk of damage and loss when the finished lens blank fell from the "handle" during machining or, conversely, had to be forcibly removed from the mold to which it was stuck. A critical feature of Neefe is that the lens blank be totally non-adherent to the mold cup.
In addition to the prior art deficiencies enumerated above, another serious disadvantage of the prior art arises from the extremely abrasive nature of the lens blanks produced for rigid gas permeable and soft lenses and the seriously adverse effect such blanks have on the life of the diamond tool used to machine the blank into a usable optical lens. Thus, still another unfilled need in the optics industry requires the development of means and methods to substantially reduce the amount of machining required for abrasive lens materials by current practices. It is toward the elimination of these several deficiencies in the prior art that this invention is directed.